PROJECT SUMMARY/ABSTRACT Vaccines are one of the most successful interventions in public health, preventing a variety of infectious diseases and providing a substantial health care cost savings. Even so, many parents have become concerned with vaccines and are opting out of required vaccination for their children through nonmedical exemptions. This increase in vaccine refusal rates has contributed to recent infectious disease outbreaks nationwide, causing many states to restrict immunization exemptions. In fact, California enacted immunization legislation SB277, which will remove all nonmedical exemptions. As California is the first state in more than three decades to eliminate all nonmedical exemptions we do not currently have an evidence base of the effect policy changes such as this will have at the population level. The implementation of SB277 provides an opportunity to evaluate the impact of elimination of nonmedical exemptions. Due to the size and importance of California in the policy landscape, the implementation of SB277 has the potential to guide other states immunization policy modifications. Therefore, the evaluation of California's recent immunization legislation could have national implications. Our team has previously evaluated vaccine hesitancy and refusal and the impact of immunization exemptions on disease control. Additionally, we have conducted analyses of the ease of obtaining nonmedical exemptions. Through this work, we determined states with less stringent requirements have higher rates nonmedical exemptions than the states with stricter requirements. We also conducted studies that documented the association between nonmedical immunization exemptions and increased disease incidence. Thus, our team's previous evaluation and analysis can be extended to study the effect of total elimination of nonmedical exemptions. The proposed study will develop an evidence base for the implementation of immunization exemption legislation through 1) analyzing rates, clustering, and variances of immunization exemptions before and after SB277 implementation, 2) evaluating the motivations, attitudes, beliefs, and health care practice burden relating to vaccinations in the context of SB277, 3) assessing the determinants and implications of variability in implementation and enforcement of legislation at the school level, and 4) ascertaining the impact of SB277 on the rates of home schooling. Ultimately, we will develop an evidence base for the implementation and effect of immunization legislation that has the potential to guide decision-making at the state and national levels.